In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw “flames passing rapidly downwards.” Ever since, there has been a fascination with electrically elicited visual perception. The general concepts of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthesis devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired.
In the early 1930's, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart.
As intraocular surgical techniques have advanced, it has become possible to apply stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparati to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular retinal prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide.
Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on such a device. The passage of current causes changes in electrical potentials across neuronal membranes, which can initiate neuron action potentials, which are the means of information transfer in the nervous system.
Based on this mechanism, it is possible to input information into the nervous system by coding the information as a sequence of electrical pulses which are relayed to the nervous system via the prosthetic device. In this way, it is possible to provide artificial sensations including vision.
One typical application of neural tissue stimulation is in the rehabilitation of the blind. Some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface. This placement must be mechanically stable, minimize the distance between the device electrodes and the neurons, and avoid undue compression of the neurons.
In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrode assembly for surgical implantation on a nerve. The matrix was silicone with embedded iridium electrodes. The assembly fit around a nerve to stimulate it.
Dawson and Radtke stimulated cat's retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses. These experiments were carried out with needle-shaped electrodes that penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson).
The Michelson '933 apparatus includes an array of photosensitive devices on its surface that are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a “bed of nails” having conductors which impinge directly on the retina to stimulate the retinal cells. Such a device increases the possibility of retinal trauma by the use of its “bed of nails” type electrodes that impinge directly on the retinal tissue.
The art of implanting an intraocular prosthetic device to electrically stimulate the retina was advanced with the introduction of retinal tacks in retinal surgery. De Juan, et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan, et al., 99 Am. J. Ophthalmol. 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina against the choroid and the posterior aspects of the globe. Retinal tacks are one way to attach a retinal array to the retina.
The retina is extraordinarily fragile. In particular, retinal neurons are extremely sensitive to pressure; they will die if even a modest intraocular pressure is maintained for a prolonged period of time. Glaucoma, which is one of the leading causes of blindness in the world, can result from a chronic increase of intraocular pressure of only 10 mm Hg. Furthermore, the retina, if it is perforated or pulled, will tend to separate from the underlying epithelium, which will eventually render it functionless. Thus attachment of a conventional prosthetic retinal electrode device carries with it the risk of damage to the retina, because of the pressure that such a device could exert on the retina.
Byers, et al. received U.S. Pat. No. 4,969,468 in 1990 which disclosed a “bed of nails” electrode array which in combination with processing circuitry amplifies and analyzes the signal received from the tissue and/or which generates signals which are sent to the target tissue. The penetrating electrodes are damaging to the delicate retinal tissue of a human eye and therefore are not applicable to enabling sight in the blind.
In 1992 U.S. Pat. No. 5,109,844 issued to de Juan et al. on a method of stimulating the retina to enable sight in the blind wherein a voltage stimulates electrodes that are in close proximity to the retinal ganglion cells. A planar ganglion cell-stimulating electrode is positioned on or above the retinal basement membrane to enable transmission of sight-creating stimuli to the retina. The electrode is a flat array containing 64-electrodes.
Norman, et al. received U.S. Pat. No. 5,215,088 in 1993 on a three-dimensional electrode device as a cortical implant for vision prosthesis. The device contains perhaps a hundred small pillars each of which penetrates the visual cortex in order to interface with neurons more effectively. The array is strong and rigid and may be made of glass and a semiconductor material.
U.S. Pat. No. 5,476,494, issued to Edell, et al. in 1995, describes a retinal array held gently against the retina by a cantilever, where the cantilever is anchored some distance from the array. Thus the anchor point is removed from the area served by the array. This cantilever configuration introduces complexity and it is very difficult to control the restoring force of the cantilever due to varying eye sizes.
Sugihara, et al. received U.S. Pat. No. 5,810,725 in 1998 on a planar electrode to enable stimulation and recording of nerve cells. The electrode is made of a rigid glass substrate. The lead wires which contact the electrodes are indium tin oxide covered with a conducting metal and coated with platinum containing metal. The electrodes are indium tin oxide or a highly electrically conductive metal. Several lead-wire insulating materials are disclosed including resins.
U.S. Pat. No. 5,935,155, issued to Humayun, et al. in 1999, describes a visual prosthesis and method of using it. The Humayun patent includes a camera, signal processing electronics and a retinal electrode array. The retinal array is mounted inside the eye using tacks, magnets, or adhesives. Portions of the remaining parts may be mounted outside the eye. The Humayun patent describes attaching the array to the retina using retinal tacks and/or magnets. This patent does not address reduction of damage to the retina and surrounding tissue or problems caused by excessive pressure between the retinal electrode array and the retina.
Mortimer's U.S. Pat. No. 5,987,361 of 1999 disclosed a flexible metal foil structure containing a series of precisely positioned holes that in turn define electrodes for neural stimulation of nerves with cuff electrodes. Silicone rubber may be used as the polymeric base layer. This electrode is for going around nerve bundles and not for planar stimulation.
The retina is also very sensitive to heat. Implanting a retinal prosthesis fully within the eye may cause excessive heat buildup damaging the retina. It is, therefore, advantageous to implant an electrode array on the retina attached by a cable to heat producing electronics which are implanted somewhere outside the eye. If no electronics are implanted in the eye, it is necessary to run one wire for each electrode from the electronics package to the electrode array. These wires must be extremely thin. While grouping them together in a cable with a protective sheath provides some protection, the array and cable must be handled carefully to prevent damage to the electrode array or cable.
Published US patent application 2002/0099420, Chow et al. describes a surgical tool for implantation of a retinal electrode array. The Chow device is a tube which is placed into the eye and to the implant location. Then fluid flows though the tube pushing the electrode array into place.